This application addresses broad Challenge Area (04), Clinical Research, and specific Challenge Topic 04-HL-103: assess the role of leukocyte interaction with platelets, erythrocytes, and endothelium in the pathogenesis of heart, lung, and blood diseases. Our proposal explores a new paradigm in molecular signaling and cell-cell interactions that is broadly relevant to human thrombotic and inflammatory diseases, and has the potential to generate novel therapeutic strategies and agents in addition to providing new knowledge in the field. The project unites a team of established investigators that is uniquely well-suited to examine clinically-applicable and basic questions relevant to the pathobiology of hemostasis and inflammation, builds on new discoveries that we have recently made, and poses focused aims that can be addressed expeditiously with expertise and technical capabilities that are in place in our collaborative group. The project is thus poised for rapid translational investigation and, potentially, fast track application. These features make it ideal for the Challenge Grant initiative. Thrombosis and inflammation are intricately linked in the pathogenesis of acute coronary syndromes, stroke, sepsis, acute lung injury, and a variety of other devastating human disorders. While much is known, there remain major gaps in our knowledge regarding the molecular pathways and cellular events that integrate inflammatory and hemostatic signaling in health, and mediate dysregulated signaling in disease. We have discovered a previously-unrecognized pathway that has these critical characteristics. Semaphorin-plexin D1 signaling has recently-identified roles in cellular guidance in the nervous system, but was not known to influence platelet activation, platelet-leukocyte interactions, or hemostatic and inflammatory events in vivo. Our preliminary data yield strong evidence that the semaphorin-plexin D1 signaling axis directly influences each of these, and that it may therefore be a novel target for molecular intervention. Because our current evidence indicates that semaphorin-plexin D1 signaling alters prothrombotic responses of activated platelets by modifying intracellular checkpoints and cytoskeletal organization distal to receptor-mediated cellular activation, new therapeutic agents that target this pathway could potentially be used independently or in combination with other antithrombotic therapies including cyclooxygenase inhibitors (aspirin), thienopyridines (clopidogrel), and anticoagulants (heparin, warfarin). The inter-related specific aims that we propose employ in vitro studies in informative human cell models, in vivo models carefully chosen for preclinical relevance and correlation with the in vitro experiments, and analyses of patient samples, and will rapidly advance our basic, translational, and clinical understanding of semaphorin-plexin signaling in hemostasis, inflammation, and vascular disease. The project will also form the basis for future investigations in many other experimental models and a variety of human syndromes. Our studies will exploit scientific and clinical opportunities, have the potential for broad and major impact, and will influence paradigms in diverse scientific and translational communities. Our proposal is thus directly responsive to the Challenge Grant goals and missions. PUBLIC HEALTH RELEVANCE: The investigations outlined in this proposal will address cellular and biochemical mechanisms that contribute to thrombotic and inflammatory diseases, which are major public health problems. Unregulated thrombosis (clot formation, often occluding blood vessels) and inflammation contribute directly to heart attack, stroke, sepsis ("blood poisoning"), acute lung injury, and a host of other devastating human disorders. Many gaps in our understanding of these disease and disorders remain, limiting our ability to develop new and improved therapies and preventative measures. The molecular pathway that we have discovered and will examine in this proposal is a potential target for new and novel molecular therapies. Our studies will also provide invaluable new information on how clot formation and inflammation are controlled in health, and become uncontrolled and injurious in disease.